mds: place the journaler pointer under the mds_lock

mds: place the journaler pointer under the mds_lock

When the _recovery_thread is trying to reformat the journal, it
will delete the old journal pointer and assign with a new one,
during this the mds_lock is unlocked. That means in other threads,
such as when 'flush journal', who are using the MDSLog::journaler
pointer will potentially hit use-after-free bug.

Fixes: https://tracker.ceph.com/issues/50807
Signed-off-by: Xiubo Li <xiubli@redhat.com>